Light-emitting diodes (LEDs) are used in displays for many different types of devices. The formation the LEDs is a challenge, though. For example, the use of GaN on a substrate has both a lattice mismatch and thermal coefficient of expansion (TCE) mismatch with the substrate, which can cause stresses in the structure, e.g., warping and cracks in an Si wafer. Due to the high lattice mismatch and TCE mismatch between the GaN material and the substrate (e.g., sapphire, glass, SiC, Si), defects are produced in the GaN buffer layer and subsequently in the GaN/InGaN/GaN multiple quantum wells (MQW).
More specifically, a lattice mismatch as high as 14% can exist between a GaN buffer layer and the underlying Si wafer. In addition, the TCE mismatch between the GaN LED material (e.g., GaN buffer layer) and the underlying substrate material, e.g., Si, can be at least 2×. And, due to this large lattice mismatch, a temperature ramp up can expand the GaN buffer layer about 2× more than the underlying Si material. Similarly, as the temperature ramps down, the GaN buffer layer can contract about 2× more than the underlying Si material. This mismatch will result in bowing and cracking of the GaN buffer layer and subsequently in the GaN/InGaN/GaN multiple quantum wells (MQW).
In an attempt to solve these problems, a thick layer of AlN/GaN buffer growth (e.g., on the order of 3 μm to 8 μm) has been grown on the underlying substrate, prior to the formation of the MQWs. However, it has been found that residual stress can still remain in the buffer layer, which can affect the growth of the MQW. This residual stress in the buffer layer not only creates defects in the MQW but can also change the band structure (emission color) of the MQW.